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This change implements a tweak to the Web Animations spec:
https://github.com/w3c/web-animations/commit/a9ba51338ed09170d16c47317f8e4e2eef122a82

Negative end delay now affects the progress used in the after phase.

Review-Url: https://codereview.chromium.org/2030843002
Cr-Commit-Position: refs/heads/master@{#407690}